1. Field of the Invention
The present invention relates to an apparatus for synchronously providing and selecting an oscillator from a plurality of oscillators to act as the system clock.
2. Description of the Related Art
In computer systems where oscillators are used as a clock source, it is very helpful to change the clock frequency to detect any marginal timing problem. One such method is to provide several oscillators and a controller to select one of them as the system clock source. The controller receives each of the outputs of the oscillators and also a select signal for each of the oscillators for controlling which oscillator is to be used. The controller interconnects the gate means for each of the oscillators such that no two oscillators may be turned on at the same time and to allow additional synchronization means to synchronize the system controller to the new oscillator before the new oscillator output is supplied as the clock signal for the system.
In large computer systems, the controller will typically be implemented in large scale integrated circuits. To facilitate the testing of these chips, usually the chip tester needs to treat the clock input pins as special cases because of the latches used to synchronize the new oscillator. This special treatment includes the ability to issue small pulses having a high bandwidth resulting in an increase in costs of the tester itself. Therefore, usually only a limited number of pins on the chip tester can be used as clock input pins. The number of clock input pins is a function of a number of oscillators that can act as the clock for the system. The user therefore has the option of increasing the cost of the chip tester by providing the necessary pin connection to test the plurality of oscillators used by the controller or to reduce the test coverage which is undesirable.